It is known in gas turbine engine technology that improvements in bypass turbofan engine performance can be obtained by efficiently mixing fan bypass air with core engine exhaust gases and discharging the combined flow through a single exhaust nozzle. One exhaust system for mixing the fan bypass air and exhaust gases includes a lobed mixer disposed downstream of the core engine. The lobed mixer forces the relatively hot exhaust gases to mix with the relatively cold fan bypass air for realizing improved thermodynamic performance and, thereby, improved specific fuel consumption.
Improved thermodynamic performance of the gas turbine engine occurs when the combined fan and core engine exhaust flow has a relatively uniform temperature substantially lower than the unmixed peak temperatures at the exhaust nozzle exit plane. Various exhaust system geometric parameters have been evaluated for increasing mixing effectiveness for obtaining more uniform temperature distributions. Some of the parameters evaluated include, for example, the number of lobes in the mixer, mixer height, mixer length, mixer cross sections, mixer and end view shapes, and mixer cutbacks. Additional geometric parameters evaluated include exhaust system tailpipe diameter, shape, and mixing length.
However, it has been determined that although mixing effectiveness can be increased by appropriately varying these geometric parameters, parasitic pressure losses associated with the exhaust system generally increase as well. For example, in a high bypass ratio turbofan engine, the exhaust dynamic pressure is generally a large proportion of the exhaust total pressure, and therefore the engine is subject to relatively large parasitic pressure losses. Accordingly, prior art exhaust systems typically represent compromise systems limited in mixing effectiveness by the parasitic pressure losses.
Accordingly, it is one object of this invention to provide an improved exhaust system for a mixed flow gas turbine engine.
Another object of this invention is to provide an improved exhaust system which provides increased internal mixing of core engine exhaust gases and fan bypass air to achieve a more uniform temperature distribution across the exhaust nozzle exit plane.
Another object of this inventiion is to provide an improved exhaust system having increased mixer effectiveness without significantly increasing pressure losses attributable thereto.